Owing to distinguished charging and discharging characteristics and overcharging and over discharging characteristics and repeated use with a long life, alkali cells are widely used in electronics equipment of remarkably smaller sizes and lighter weights. Characteristics of such alkali cells are largely dependent also on characteristics of non-woven fabrics for alkali cell separator.
The following conditions are required as general functions of non-woven fabrics for alkali cell separators.
(1) They can physically separate a positive pole and a negative pole from each other, PA1 (2) They have an electrical insulatability high enough to prevent occurrence of any short circuit, PA1 (3) They have a sufficient resistance to an electrolyte solution, PA1 (4) They have a sufficient resistance to electrochemical oxidation, PA1 (5) They have a low electrical resistance in an electrolyte solution impregnated state, PA1 (6) They can be readily wettable with an electrolyte solution and have a high electrolyte solution retainability, PA1 (7) They have strength and rigidity high enough to withstand during the cell assembling step, PA1 (8) They will not discharge any harmful substance to the cells, and PA1 (9) They have a distinguished permeability of oxygen gas generated at the positive pole when charged.
Accordingly, non-woven fabrics made by a dry process from polyamide fibers of 6-nylon, 6,6-nylon, etc., or polyolefin-based fibers of core-and-sheath type, whose core member is made from polypropylene and whose sheath member from polyethylene, have been so far used as non-woven fabrics for alkali cell separators.
Non-woven fabrics for alkali cell separators formed by the dry process have had large fluctuations in areal weight, and thus non-woven fabrics having a larger areal weight have been formed, followed by crushing through a hot calender before use. Such non-woven fabrics have had a high mechanical strength and a distinguished cell processability, whereas there has been such a problems as an incapability to lower the areal weight. In recent attempts to meet the requirements for a higher cell capacity by increasing the amount of electrode active materials or lowering the areal weight of separator to make the separator thinner, such problems have arisen as occurrences of detached active materials transfer and reduction of electrolyte solution retainability.
As a means of solving these problems, an alkali cell separator made from melt-blow non-woven fabrics has been proposed. The melt-blow non-woven fabrics comprise very fine fibers and thus it is possible to make the non-woven fabrics smaller pore diameters or a higher void ratio. That is, in spite of the alkali cell separator as made thinner to some extent, transfer of detached active materials can be prevented and the desired the electrolyte solution retainability can be maintained.
However, the alkali cell separator made from the melt-blow non-woven fabrics is made from finer melt-spun fibers finer obtained by injecting a high speed hot air stream to the fibers from both sides while maintaining the melt-spun state, and collected on a screen. Thus, owing to their so low mechanical strength and still so large fluctuations in the areal weight, the melt-blow non-woven fabric has such problems that limit to lowering the areal weight cannot be made smaller than some limit and it is difficult to conduct rapid charging, because of low gas permeability, that is, an incapability to release a large amount of generated reaction gas and a consequent increase in the inside pressure of the cell, when the amount of electrode materials is to be increased.
As a means of solving these problems, JP-A-5-182654 proposes a cell separator made by integrating a melt-blow non-woven fabric with a hydroentangled non-woven fabric obtained by hydroentangling treatment of a single short fiber web or with a hydroentangled non-woven fabric obtained by hydroentangling treatment of a single short fiber web and a single meet-flow webs together, as laid upon each other, by hot press fusion lamination.
As measures against formation of perforations through the melt-blow non-woven fabric and consequent enlargement of maximum pore diameter in case of integration by lamination of a melt-blow non-woven fabric and a short fiber non-woven fabric through injection of high pressure water, the above-mentioned cell separator can be prevented from the transfer of electrode active materials and can be improved in the electrolyte solution retainability by the presence of the melt-blow non-woven fabric layer and also can be improved in the strength, gas permeability and electrolyte solution retainability by the presence of the hydroentangled non-woven fabric layer. However, for the stable, industrial-scale production of such cell separators, the areal weight must be inevitably made larger throughout the entire separator. When it is tried to lower the areal weight, productivity will be considerably lowered. Thus, mass production of such cell separator is still at a premature stage.
JP-A-7-29561 proposes a cell separator made by blending short fibers of splittable composite fibers comprising a polyolefin polymer and an ethylene/vinyl alcohol copolymer with heat-fusible composite fibers and short fibers of rigid fibers having larger deniers than those of very fine fibers resulting from splitting of the splittable composite fibers and also than those of the heat-fusible composite fibers, followed by a wet web-forming process, treatment of the resulting wet process-formed non-woven fabric with high pressure water stream and hot calender finishing, and also proposes a process for producing the same.
The above-mentioned cell separator can be endowed with a mechanical strength by hot fusion of the blended heat-fusible fibers and three-dimensional entanglement of member fibers through the high pressure water stream treatment, and furthermore can be endowed with some electrolyte solution retainability by splitting the splittable fibers into finer fibers through the high pressure water stream treatment.
However, in the high pressure water stream treatment of the wet process-formed web, streaks of entanglement remain on the wet process-formed web and also perforations are formed therethrough, resulting in enlargement of maximum pore diameters and consequent failure to completely prevent the active materials from transfer.
Heat fusion of heat-fusible fibers will reduce the electrolyte solution-retaining space, resulting in lowering the solution retainability and deterioration of solution absorbability as drawbacks.
Thus, to completely prevent the active material from transfer and ensure the electrolyte solution retainability, there is no other way than to increase the areal weight of the wet process-formed web. One example disclosed in the prior art shows an areal weight of about 70 g/m.sup.2. This shows that the areal weight is hard to reduce.
In view of the foregoing problems, an object of the present invention has been made to solve the problems and provide a non-woven fabric for an alkali cell separator, which can prevent active materials from transfer and has distinguished electrolyte solution absorbability and retainability in spite of a low areal weight, and also provide a process for producing the same.